One example of a conventional received light intensity measuring device of this type is shown in FIG. 3. A photosensor 1 (which is made up of a photodiode in this example) is a conversion element that provides a photoelectric current in corresponding to a received light intensity L. A capacitor 2 connected in parallel with the photosensor 1 is discharged when the photoelectric current i flows in the photosensor 1. A signal obtained by integration of the photoelectric current is applied, as a signal V.sub.1, to the positive (+) input terminal of a comparator 3. Before the sensing operation, a transistor 4 is temporarily rendered conductive by a reset input so that the capacitor 2 is charged to a level V.sub.DD. The voltage V.sub.1 applied to the comparator 3 can be represented by the following equation (1): EQU V.sub.1 =1/C.intg.idt (1)
where C is the capacitance of the capacitor 2.
The operation of the circuit shown in FIG. 3 will be described with reference to a time chart shown in FIG. 4. A reset signal as shown in FIG. 4 is applied to the transistor 4 so that the capacitor 2 is charged and V.sub.1 is set to zero (V.sub.1 ="0"). This causes the output of the comparator 3 to be set to "0". Under this condition, the time interval t.sub.s which elapses from the time instant that discharging the capacitor 2 commences until the output of the comparator 3 is raised to "1"; that is, until the input V.sub.1 of the comparator 3 reaches a reference level V.sub.ref is measured. The time interval t.sub.s thus measured represents the intensity L of the received light.
If it is assumed that the current i is substantially proportional to the intensity L; that is, i=A L, then the input voltage V.sub.1 of the comparator 3 can be represented by the following equation (2): EQU V.sub.1 =A L t/C (2)
The period of time t.sub.s required for V.sub.1 to reach V.sub.ref was described above. Therefore, the period of time t.sub.s can be obtained from the following equation (3): EQU t.sub.s =C V.sub.ref /A L (3)
In the above-described case, the intensity L of the light received changes generally in a wide range. If the maximum value of the intensity of the light received is 10.sub.6 times as large as the minimum value, then the maximum value of the conversion time t.sub.s is 10.sub.6 times longer than the minimum value. That is, when the minimum conversion time is one (1) micro-second, the maximum conversion time will be one (1) second. This is not practical. When it is required to shorten the conversion time t.sub.s when the received light intensity L is small, the value V.sub.ref should be decreased. However, since the comparator does not work for an input close to the supply voltage (V.sup.+ and V.sup.-) because of its fundamental characteristic, if the reference voltage V.sub.ref is made extremely close to the ground level of the comparator, then the circuit of FIG. 3 will not operate. This difficulty may be eliminated by a method in which a power source is additionally provided for the comparator only and the value V.sup.- is set to a minus potential. However, the method is not applicable to the case where the circuit should have the common ground level in its entirety. Furthermore, in the case where it is possible to set the reference value V.sub.ref to a considerably small value, if the value V.sub.ref is fixed, then in the case where the received light intensity L is large, the response time t.sub.s is decreased to a point of making it impossible for other circuits to function properly.